In a liquid crystal display device that is provided on a cell-phone or a personal digital assistance, a backlight apparatus for illuminating a display portion to be bright is used. This backlight apparatus is composed of a light guide plate to be arranged on a back face of a liquid crystal panel representing a display section; a light source such as a light emitting element (which is LED) and a cold-cathode tube to be arranged on the side of the light guide plate; and a light guide element for guiding light coming from the light source to the light guide plate. This backlight apparatus equipped with the aforesaid structure has an advantageous point that the total apparatus can be made to be thin, because light enters through a side face of the light guide plate, and light sources are not required to be arranged in the direction of a thickness of the light guide plate.
In recent years, liquid crystal display devices are installed in a thin type cell-phone and a digital camera in many cases, and a demand for a small-sized backlight apparatus has become strong. Responding to this trend, a light guide plate is becoming thinner. However, there are actual circumstances that it is difficult to reduce the size of LED, which is used as a light source, to the same extent as a thickness of the light guide plate. On the other hand, directivity in the light-emitting property of LED is generally low, and some outgoing beams are diverged radially at a wide angle. Therefore, how to cause the outgoing beam emitted from LED to enter the thin light guide plate efficiently is a problem.
Japanese Patent Publication Open to Public Inspection No. 2003-121840 discloses a backlight apparatus provided with a light-receiving section on which an inclined plane that is inclined at an angle of 45° or less upward from the main body portion of the light guide plate is formed. Further, it discloses a technology to attain a thin backlight apparatus by providing a light-receiving surface inclined to be substantially perpendicular to the inclined plane, and by inclining so that a light-emitting surface of the light source may become to be parallel with this light-receiving surface.
In the case of the former, however, a reflecting film is coated on a slope for increasing the reflectance on the slope, because a beam entering the slope is not always subjected to total reflection. Thus, the manufacturing process for the light guide plate is made to be complicated by the process of coating of the reflecting film. Further, there is also a problem that the utilization efficiency of light is lowered, because no small amount of light is absorbed by the reflecting film due to the property of the reflecting film. Further, in the case of the latter, there is also a problem that the utilization efficiency of light is lowered when a further thinner main body portion of the light guide plate is provided and the incident light reflects larger times at the light-receiving section in such a structure, although the technology in the case of the latter can contribute to provide the thinner backlight apparatus.